Violent crime has plummeted in the Washington area and in major cities across the country, a trend criminologists describe as baffling and unexpected.

The District, New York and Los Angeles are on track for fewer killings this year than in any other year in at least four decades. Boston, San Francisco, Minneapolis and other cities are also seeing notable reductions in homicides.

“Experts did not see this coming at all,” said Andrew Karmen, a criminologist and professor of sociology at the John Jay College of Criminal Justice in New York.

In the District and Prince George’s County, homicides are down about 17 percent this year.

Criminologists have different theories about why crime is down so much, although many agree that the common belief that crime is connected to the economy is false.